Many individuals with spinal cord injury cannot generate the muscle torque necessary for efficient locomotion even after extensive conventional rehabilitation efforts. This may be attributed to both neural and muscular declines after injury. However, with task specific training that maintains appropriate peripheral sensory inputs, the ability to step and stand can be reacquired by cats and rats even after a complete thoracic spinal cord transection. This potential for neural plasticity also exists in human lumbosacral neural circuits. Spinal cord injured patients who have no voluntary motor control or sensation below the level of the lesion can generate step like efferent patterns when suspended over a moving treadmill belt with partial body weight support and manual assistance. Further, peripheral sensory modalities such as limb load, joint position and treadmill speed modulate the EMG activity of extensors and flexors during stepping on the treadmill by interacting with lumbosacral neural circuits. Repetitive step training may have enduring effects on the capacity of these neural networks and has the potential to reduce the effects of muscle disuse. In the proposed experiments, we will determine the responsiveness of the human lumbosacral spinal cord after a clinically complete thoracic injury to sensory inputs related to limb load, the kinematics of the phase of the step cycle, and hip position during stepping with body weight support on a moving treadmill. In addition we will demonstrate whether training with these inputs can optimize the level of motor pool activity and increase muscle mass and therefore the potential for locomotion. The proposed studies will employ measures of kinematics, loading and electro myographic activity of the lower limbs. These studies will provide a better understanding of neural mechanisms that are available in the human spinal cord to generate stepping. They will demonstrate aspect of use-dependent plasticity that are being studied in the animal models of this PPG. It is anticipated that the findings from the proposed studies will facilitate the development of new rehabilitation strategies for optimizing the recovery of mobility following neurologic injury.